Various kinds of optical disks shaped like discs for optically recording and/or reproducing information are put into practical use as optical recording and reproducing mediums. A read-only optical disk having embossed pits formed on a disk substrate beforehand, a magneto-optical disk for recording data by magneto-optical effect and a phase-change optical disk for recording data by phase-change of a recording film are available as such optical disks.
Of these optical disks, in the optical disk in which data can be written, such as a magneto-optical disk and a phase-change optical disk, it is customary to form grooves extending along recording tracks on a disk substrate. The grooves are so-called guide grooves formed along recording tracks in order to make mainly tracking servo, and an opening end between the grooves is referred to as a land.
In the optical disk with the grooves being formed thereon, it is customary to make tracking servo by using a tracking error signal based upon a push-pull signal obtained from light reflected and deflected on the grooves. The push-pull signal is calculated as a difference between outputs of two photo-detectors located symmetrically across the center of the track, for example, after the two photo-detectors have detected light reflected and diffracted on the groove.
In these optical disks, high recording density has been achieved so far by improving reproduction resolution of an optical pickup mounted on a reproducing apparatus. Then, improvement of the reproduction resolution of the optical pickup has been optically realized so far by reducing a wavelength λ of laser light for use in mainly reproducing data or by increasing a numerical aperture NA of an objective lens for converging laser light on the optical disk.
In the respective conventional formats of a so-called CD-R available as a write-once type CD (Compact Disc), an MD (Mini Disc) available as a rewritable magneto-optical disk, a DVD-R available as a write-once DVD (Digital Versatile Disc) or a DVD+RW or DVD-RW available as a rewritable DVD (these trade names are all registered trademarks of optical disks), the groove widths most suitable for the recording and reproducing characteristics are different depending upon factors such as properties of recording films and characteristics of servo signals.
In the ordinary optical disk manufacturing process, when a stamper for use in molding its substrate is manufactured, a photoresist is coated on a master substrate and the above-mentioned pits and grooves are formed by so-called photolithography using pattern exposure and development. Hence, the groove width is determined by a diameter of a beam spot of exposure beam.
When a master of an optical recording and reproducing medium such as the above-mentioned CD-R and CD-RW is recorded by one exposure beam, pattern exposure is made by an optical recording apparatus of which schematic arrangement is shown in FIG. 13. In FIG. 13, reference numeral 20 denotes a light source formed of a He—Cd laser of gas laser using gas, for example, as an amplification medium. Laser light L emitted from this light source 20 is deflected 90° in its traveling direction by a mirror M1 and introduced into a modulation optical system 25. In the optical modulation system 25, the laser light L is reduced in diameter of beam by a condenser lens L1 and introduced into an AOM (Acousto Optical Modulator; acousto-optic modulator) 28, in which it is modulated in light intensity in response to ultrasonic waves that were modulated based upon a recording signal supplied to the AOM 28. Reference numeral 27 denotes a driving driver for inputting a signal such as an EFM signal.
The laser light L modulated by this AOM 28 is enlarged or reduced in beam diameter by a beam enlargement lens or a beam reduction lens L2, is traveled as the parallel beam and reflected by a mirror M2, thereby being introduced into a moving optical table 40 in the horizontal direction.
The moving optical table 40 includes a lens L3, for example, as a focusing and diffraction light correction optical system, a mirror M3 for directing the direction of the optical axis to the irradiated surface and an objective lens L4. The lens L3 is located on a light incident side convergence surface P2, formed at the position conjugating to the focusing condenser surface P1 of the objective lens L4, at its position in which the laser light L is to be focused.
Thereafter, the laser light L is focused on the surface of a photoresist 12 on the master substrate 11 through this focusing and diffracted light correction lens L3 and the objective lens L4 and thereby the photoresist is exposed with a predetermined pattern. The master substrate 11 is rotated as shown by an arrow b by a rotary drive means, though not shown. A dot-and-dash line c denotes a center axis of the substrate 11.
In such optical recording apparatus, the above-mentioned beam relay optical system is located between the light source 20 and the objective lens L4 to change the focal distance of the lens L2 or the lens L3 such that the objective lens L4 may focus light on the photoresist 12 and that the effective numerical aperture NA relative to the objective lens L4 may change to change the diameter of the exposure beam.
In the above-mentioned CD-R and CD-RW, concave and convex patterns of the groove are recorded by a He—Cd laser (wavelength is 442 nm), and an optimum groove width falls within a range of from 550 nm to 600 nm. Since the DVD+RW, DVD-R and DVD-RW of the high density optical disks have a recording capacity of 4.7 GB, which is high recording density as high as about 7.2 times the recording density of the CD-R and CD-RW, the optimum groove width thereof is smaller than that of the above-mentioned CD-R and the like and falls within a range of from 300 to 330 nm. Therefore, by using a Kr laser (wavelength is 413 nm) with short wavelength, the spot diameter d of exposure beam can be reduced, and hence the optimum groove width of the DVD+RW, DVD-R and DVD-RW can be realized.
The spot diameter d of the exposure beam is expressed by the following equation (1):d=1.22×λ/NA  (1)
(λ: recording wavelength, NA: numerical aperture)
In a cited patent reference 1 (official gazette of Japanese laid-open patent application No. 10-241214), a groove width that falls within a range of from about 600 nm to 800 nm can be realized by using an Ar laser (wavelength is 458 nm).
Recording wavelengths λ, track pitches, groove widths and ratios between the groove width and recording wavelength of the above-mentioned respective optical disks are shown on the following table 1.
TABLE 1GrooveRecordingTrackGroovewidth/Rec.wavelength λpitchwidthwavelengthCited patent458 nm600-800nm1.31-1.75reference 1CD-R442 nm1600nm600nm1.36CD-RW442 nm1600nm550nm1.24DVD+RW413 nm740nm300nm0.73DVD-R413 nm740nm330nm0.80DVD-RW413 nm740nm310nm0.75
A study of this table 1 reveals that the ordinary density optical disks of CD-R and CD-RW have the groove widths larger than the recording wavelength (442 nm), i.e., the ratios between the groove width and the recording wavelength larger than 1.0, which is enough to expose, i.e., sensitize the photoresist in most part of the spot of exposure beam so that these groove widths can be formed relatively easily.
However, the high density optical disks of the DVD+RW, DVD-R, DVD-RW and the like have the groove widths smaller than the recording wavelength (413 nm), i.e., the ratios between the groove width and the recording wavelength smaller than 1.0 so that these groove widths cannot be formed relatively easily.
Further, with respect to the high recording density optical disks, there is proposed a format by which a groove width can be much more microminiaturized up to approximately less than 200 nm. In a DVR (Digital Video Recordable) that is under development as an ultra-high density optical disk, as FIG. 14 shows a schematic plan arrangement in a partly enlarged-scale, its format is discussed such that a groove is formed as a wobble groove, a track pitch being selected to be 325 nm and a groove width being selected to be about 150 nm. However, there has not yet been proposed an ultra-high density optical disk manufacturing method which is not only excellent in productivity in actual practice but also satisfactory in yield. In FIG. 14, reference numeral 2 denotes a groove and reference numeral 8 denotes a land.
A cited patent reference 2 (Japanese patent No. 3104699) has reported a molded substrate having a groove width less than 100 nm manufactured by a manufacturing method in which a land portion and a groove portion are inverted by using a so-called mother stamper whose concavities and convexities are inverted to those of a stamper by a duplicate of a stamper.
However, the example described in the above-described cited patent reference 2 has an extremely large land width as compared with a groove width. The following table 2 shows groove widths, land widths, track pitches and ratios between groove width and track pitch of inventive examples 1 to 3 of this cited patent reference 2 and the CD-R, CD-RW, DVD+RW, DVD-R, DVD-RW and MD, respectively.
TABLE 2GrooveLandTrackGroove width/widthwidthpitchTrack pitchCited patent40nm360 nm400nm0.10reference 2Inventiveexamples 1, 2Cited patent60nm290 nm350nm0.17reference 2Inventiveexample 3CD-R600nm600 nm1600nm0.38CD-RW550nm600 nm1600nm0.34DVD+RW300nm440 nm740nm0.41DVD-R330nm410 nm740nm0.46DVD-RW310nm430 nm740nm0.42MD1200nm400 nm1600nm0.75
As is clear from the above-described table 2, in the above-described cited patent reference 2, since the groove width is extremely small as compared with the land width, accordingly, the ratio between the groove width and the track pitch is as very small as 0.10 to 0.17 and amplitude amounts of a push-pull signal serving as a tracking servo signal and a cross-track signal (Cross Track Signal: CTS) decrease, stable tracking servo cannot be realized. Therefore, this conventional manufacturing method cannot be directly applied to an optical disk that intends to increase recording density by reducing the track pitch to less than about 350 nm.
The amplitude amount of the push-pull signal is maximized when the ratio between the groove width and the track pitch is ½, and the amplitude amount of the CTS signal is maximized when the ratio between the groove width and the track pitch is approximately ⅓ or approximately ⅔. As shown on the above-described table 2, in the commercially-available optical disks such as the CD-R, CD-RW, DVD+RW, DVD-R, DVD-RW, the ratio between the groove width and the track pitch falls within a range of from approximately 0.34% to 0.75%.
Moreover, the above-described cited patent reference 2 can realize the narrow groove width by using the inverted pattern in which an area which might serve as a future land portion is formed as the groove by the mother stamper as described above. In this case, when recording light is wobbled and the pattern is exposed to form a wobble groove, since a portion that might be formed as a future land portion is formed as a groove, different wobble signals are recorded on the left and right of the groove. There is then a risk that a leakage of a signal will occur when a wobble signal is reproduced. Thus, a problem arises, in which it will be difficult to form wobbling grooves at a level suitable for actual practice.
In view of the aforesaid aspects, it is an object of the present invention to provide an optical recording and reproducing medium, an optical recording and reproducing medium manufacturing stamper and its manufacturing method suitable for the application to high recording density optical disks such as the aforementioned DVR and which is excellent in tracking servo characteristics, in reproducing characteristics of wobble signals and which can achieve high recording density at a level suitable for actual practice.